Method and apparatus for recording digital images on photosensitive material

ABSTRACT

The present invention is directed to a method and apparatus for exposing photosensitive material to form high quality continuous tone and/or color images thereon. The preferred apparatus includes an imaging head comprised of a plurality of red light sources, a plurality of green light sources, and a plurality of blue light sources. The light produced by said green light sources is passed through a first filter having a narrow spectral transmission characteristic in the green spectral range. Similarly, the light produced by said blue light sources is passed through a second filter having a narrow spectral transmission characteristic in the blue spectral range.

RELATED APPLICATIONS

[0001] This application is a continuation of U.S. application Ser. No.09/303,258 filed on Apr. 30, 1999 which claims priority based onprovisional application 60/083,975 filed May 1, 1998.

FIELD OF THE INVENTION

[0002] This invention relates generally to a method and apparatus forproducing high quality continuous tone and/or color images onphotosensitive material (i.e., photographic paper or film) frominformation provided in digital form.

DESCRIPTION OF PRIOR ART

[0003] In the field of photographic digital printers and image setters,the use of multiple light sources to create individual pixels is wellknown. U.S. Pat. No. 3,988,742 describes using LEDs and fiber opticlight guides to deliver the light to the photosensitive material.Applications of this technology have included type setting, and thegeneration of lithographic films for printing. In these applications,the light output of LEDs is coupled into the input end of the fiberoptic tubes. The output ends of the fiber optic tubes are arranged in alinear array. As photosensitive material is passed by the linear arrayof fiber optic tubes, the LED's are illuminated in such sequence as tocause the formation of indicia or images on the photosensitive material.This process is described in U.S. Pat. Nos. 3,832,488 and 4,000,495 and5,093,682. The use of fiber optic tubes of both square and round crosssections is known.

[0004] In such applications as described above, the precision assemblyof the output ends of the fiber optics is important. Poor alignment, oruneven spacing of the output ends of the fibers cause distortions in theimages being generated. U.S. Pat. Nos. 4,364,064 and 4,389,655 describedevices for precisely positioning the fiber optic tubes. U.S. Pat. No.4,590,492 describes a method for masking the ends of the fiber optictubes to provide more precise alignment of the light sources exposingthe photosensitive material.

[0005] Prior art systems have typically been used to form lithographicimages and indicia for typesetting and printing consisting solely ofwhite and black areas without intermediate tones. Due to the lack ofintermediate tonal detail, such systems are tolerant of some imprecisionin the quality and quantity of light delivered to the photosensitivematerial. More particularly, they are typically tolerant of imperfectpixel to pixel alignment because of slight pixel blooming which occursas a consequence of using exposure levels high enough to saturate thephotosensitive material.

[0006] Continuous tone images, e.g., images which are predominatelycomposed of middle tones, whether colors or gray tones, requiresignificant precision in pixel formation and alignment. Misalignment ofone pixel relative to its neighbors will cause unwanted lines or otherartifacts to appear in a continuous tone image.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to a method and apparatus forexposing photosensitive material to form high quality continuous tone,color images thereon.

[0008] Embodiments of the invention are typically comprised of animaging (or print) head comprised of multiple pixel image generators,e.g., light sources. The head is preferably mounted for linear movementin a first, i.e., scanning direction, across the width of a web ofphotosensitive material. The photosensitive material is mounted forlinear movement in a second direction perpendicular to said firstdirection to enable successive scan strips (i.e., groups of scan lines)to be imaged onto said photosensitive material. The portion of the webto be printed can be referred to as an “image field” and can beconsidered to consist of a rectangular matrix of “rows” extending in thescan direction across the web width and “columns” extendingperpendicular to the rows, i.e., longitudinally along the web. The headcan be stepped or moved continuously in the scan direction with themultiple light sources being selectively enabled to expose an image ontothe photosensitive material.

[0009] A system in accordance with the invention produces a field of“interpolated pixels”, each interpolated pixel being formed by theoverlap between adjacent pixel images. More particularly, aninterpolated pixel in accordance with the invention can be formed by theoverlap between adjacent pixel images displaced in the scan direction,e.g., horizontal, and/or by the overlap between adjacent pixel imagesdisplaced in the longitudinal direction, e.g., vertical.

[0010] It is an object of this invention to provide a method andapparatus for precisely delivering light to photosensitive material toallow the printing of extremely high quality continuous tone images fromdigital information.

[0011] It is a further object of this invention to provide a method andapparatus for blending the pixels of an image presented in digital form,so as to increase the apparent resolution and sharpness of the resultingprinted image.

[0012] It is a further object of this invention to provide a low costimaging head capable of precisely delivering light to photosensitivematerial with the precision required to allow the printing of extremelyhigh quality continuous tone images.

[0013] In accordance with a preferred embodiment of the invention, theimaging head is comprised of square or rectangular pixel imagegenerators, e.g., fiber optic tubes, mounted to form a rectangular array. The pixel image generators are inclined at an angle of 45 degrees tothe scan direction. As the print head scans, each fiber optic tube canexpose a pixel image onto the photosensitive material. The exposurelevels of the pixel images are preferably specified in a digital filerepresenting an image to be printed. As a result of being inclined at 45degrees, the pixel images exposed onto the photosensitive material arediamond shaped. Each pixel image overlaps its neighbor by substantially50% of the center to center distance between pixel images. The patternresulting from the overlapping of the pixels images generatesgeometrically interpolated pixel areas with each such area being about25% of the original pixel image area. Further, the shape of each pixelrelative to the scan direction causes the exposure in the area betweenadjacent scan lines to remain consistent and to cause superior blendingof each pixel image with its neighboring pixel images.

[0014] The generation of high quality continuous tone images requiresthe precise blending of the pixels imaged on the photosensitivematerial. Precise blending of the pixels requires that the pixelsthemselves be of uniform color and intensity. Fiber optic tubes operateby the principle of “total internal reflection” of the light wavesintroduced into the fiber optic tube. The symmetric nature of roundfiber optic tubes is such that the image of the light source at theinput end of the tube is delivered, more or less intact, to the outputend of the tube. The nature of LEDs and other light sources is that thelight emitting element does not emit perfectly uniform illumination.Consequently, the image of the LED die will propagate down a round fiberoptic tube and be delivered to the end. The non-uniform nature of theimage of the light element will cause the pixels to blend poorly withone another. In a fiber optic tube of square or rectangular crosssection, the image is scrambled by the successive reflections off theorthogonal walls of the tube. The principles of “total internalreflection” causes most all of the light energy which enters the fiberoptic tube to be delivered to the output, but with the image scrambledto such an extent as to make the light output from the fiber optic tubesubstantially uniform. This uniformity is desirable to achieve thehighest quality of continuous tone images.

[0015] As described above, the generation of high quality continuoustone images requires extreme precision in the placement and uniformityof illumination of the pixels comprising the image. Imprecision ineither of these will result in artifacts or lines appearing in theprinted image. In the printing method as described herein, a pluralityof independently excitable light sources is employed to provide theillumination for a matching number of pixels. When printing onto colorphotosensitive materials, extreme precision is required in the matchingof the spectral output characteristics of the multiple light sources. Ifthe light sources are not of precisely the same spectralcharacteristics, artifacts or lines will appear in the printed image.Light sources of different spectral characteristics will exposedifferent layers of the photosensitive material with differing efficacy.It is possible to adjust the intensity of light sources to be equallyeffective in exposure at a particular color or shade. However, if thespectral characteristics of the light sources are not precisely matchedto one another, they will not be equally effective at exposing adifferent color. The result will be that artifacts or lines appear insome colors of the printed image, but not others.

[0016] In a preferred embodiment of the invention, the individual lightsources are matched in spectral output with the use of a narrow passband filter for each color. The narrow pass nature of the filterrestrains the exposing energy of each LED to a narrow wavelength rangewithin which the photosensitive material will have uniform colorresponse. The filter is fabricated in such a way as to cover all of thepixels of a given color with the same filter. In the embodiment of theinvention actually constructed by the inventor, filters of twoindependent wavelengths were fabricated on the same substrate and placedover the ends of the fibers.

[0017] The print head can be imaged onto the photosensitive materialeither by intimate contact or via a lens system. In one implementedembodiment of the invention, the print head is comprised of threecolumns of fiber optic tube ends, each column containing 32 tube ends.The head scans across a 30 inch width of photosensitive material andexposes a strip of approximately 0.100 inches during each scan. Aftereach scan, the material is advanced in the longitudinal direction by theheight of the exposed strip area. The head then successively scansacross the photosensitive material exposing additional strip areas tofully cover the image field. In an alternative embodiment of theinvention, the head could be the full width of the material obviatingthe need for the head to scan across the width of the material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic isometric illustration of a print head inaccordance with the invention depicting a rectangular array of squarefiber optic tube ends;

[0019]FIG. 2 is a schematic isometric illustration of a portion of adigital printer in accordance with the invention showing a drum fortransporting a photosensitive material web and a print head mounted forlinear movement to scan across the width of the web;

[0020]FIG. 3 is a schematic illustration depicting a prior art schemefor enabling a print head comprised of a column of square light sourcesto scan across a web of photosensitive material to expose pixel imagesthereon;

[0021]FIG. 4A is a schematic illustration depicting a preferred systemin accordance with the invention for exposing pixel images onto thephotosensitive material; and FIG. 4B schematically depicts analternative head configuration;

[0022]FIGS. 5 and 6 are schematic illustrations showing howlongitudinally displaced pixel images overlay in the invention to forminterpolated pixels;

[0023]FIG. 7 is a schematic isometric illustration depicting a colormatching filter for installation on the print head;

[0024]FIG. 8 depicts typical spectral characteristics of red, green andblue LED light sources;

[0025]FIG. 9 depicts a typical range of spectral characteristics of agroup of LED light sources; and

[0026]FIG. 10 shows the spectral transmission characteristics of thecolor matching filter shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027]FIG. 1 shows the output end of an imaging or print head 1 inaccordance with a preferred embodiment of the invention. The head iscomprised of a plurality of pixel image generators, e.g., light sourcesdefined by fiber optic tubes. The tubes are preferably of substantiallysquare or rectangular cross section and are arranged in columns 5, 6, 7.The fiber optic tubes are selected or manufactured to be of precisecross sectional dimension. With the aid of an assembly fixture, thefiber optic tubes are clamped by frame 4 or bonded in place. Each columnis comprised of multiple fiber optic tube output ends which areassembled and positioned in precise alignment with one another. Thecolumns may be arranged in contact, or spaced apart with precisionspacers 8. In either configuration, each column contains the same numberof fiber optic tubes and is precisely the same length. After assembly,the fiber optic tube ends are machined to a predetermined cross sectionand polished. Although the assembly as shown in FIG. 1 has a planarcross section, other shapes are possible. Specifically, curved crosssections are possible which have the advantage of being able to contourto the surface of a drum around which the photosensitive material isplaced, or to correct for focus aberrations common to lenses. The inputends of the fiber optic tubes are connected (not shown) to independentlyexcitable light sources, preferably light emitting diodes (LEDs). Theembodiment of the invention as shown uses fiber optic tubes to deliverthe light to photosensitive material and to define the shape of pixelimages exposed thereon. An alternative embodiment of the invention canemploy substantially square LED dice arranged in a matrix and mounteddirectly into frame 4.

[0028]FIG. 2 shows the head 1 in place in one possible application ofthe invention. In this application, the image of the output ends of thefiber optic tubes 3 is projected via a lens 14 onto a target surface,i.e., a web or sheet of photosensitive material 23, which is tensionedaround a drum 10. The print head assembly 15 and lens assembly 16 aremounted for linear lateral movement in a scan direction 18 parallel tothe axis of the drum 10 around which the photosensitive material 23 ispositioned. As the print head scans, encoder 24 indicates the columnarposition of the print head along the scan line. The photosensitivematerial 23 is moved longitudinally perpendicular to the scan direction18.

[0029]FIG. 3 shows a representation of a print head 37 which has beendescribed in the prior art. Light sources 38, identified as S₀ throughS_(n), are of square cross section, and expose pixel images representedin the grid 39. As the print head scans across a row of thephotosensitive material (defining an image field comprised of m columnsand n rows), a pixel image can be exposed at each columnar position P₀through P_(m). The exposures of the pixel images are enabled by nseparate enabling signals 46 E₀ through E_(n). The enabling signals aresynchronized with the encoder 24 indicated in FIG. 2.

[0030]FIG. 4A shows a representation of a preferred head 1 in accordancewith the invention. The print head is inclined at an angle of 45 degreesto the scan direction. Light sources 76 S₀ through S_(n), define adiamond shaped cross section as a result of the square ends beinginclined at 45 degrees relative to the scan direction. Light sources 76S₀ through S_(n), expose pixel images 78 as represented in the grid 80.As the print head 70 scans across the photosensitive material, the pixelimages can be exposed at each position P₀ through P_(m). As aconsequence of being inclined at 45 degrees, the pixel images associatedwith a single columnar position P are not exposed simultaneously.Instead, pixel P_(m),S₀ is exposed at the same time as pixels P_(m−1),S₁and P_(m−2),S₂, etc. The exposures of the pixel images are enabled by nseparate enabling signals 82 E₀ through E_(n). The enabling signals aresynchronized with the encoder 24 indicated in FIG. 2. As the head scans,the pixel images in accordance with the invention overlap. That is, thepixel images of each scan row overlap with the pixel images of rowsabove and below by substantially 50% of the center to center distancebetween pixel images. Also, each pixel image along a row overlaps withpixel images horizontally displaced before and after by substantially50% of the center to center distance between pixel images.

[0031] Whereas the head in FIG. 4A scans laterally perpendicular to thelongitudinal direction of the photosensitive material 23, FIG. 4Bdepicts an alternative head configuration 83 in which the head can befixed. More particularly, FIG. 4B depicts a head extending across thewidth of the photosensitive material 23. The head 83 is comprised of atleast two rows of square pixel image generators (e.g., fiber optic tubeends) oriented at a 45 degree angle with respect to the lateral andlongitudinal directions. The head 83 is capable of imaging the samepixel image pattern as is depicted in FIG. 4A.

[0032] Referring to FIG. 5, as the photosensitive material is exposed bythe pixel images, the exposure contribution from pixel image 26 to itsscan line 28 is greatest in the center portion because the pixel 26 iswidest along that line. The exposure level decreases linearly withdistance transverse to the center line 30 of the pixel. The exposurelevel decreases to the point where it is zero at the center line 32 ofthe adjacent scan line 34. Correspondingly, the exposure contributionfrom the adjacent pixel image 36 to the raster line 28 decreases withthe distance from it's center line 32. It can be seen that at any pointon a line between the center of the pixel image 26 and the center of theadjacent pixel image 36, the exposure level is comprised of a portion ofeach pixel image. Due to the shape of the pixel image, it can be seenthat the exposure level in the region between the center line 30 of thepixel image 26 and center line 32 of the adjacent pixel image 36 is alinear average of pixel 26 and the adjacent pixel 36.

[0033]FIG. 6 shows the pattern of diamond pixels 40 relative to atraditional square pixel pattern 42 as represented in FIG. 3. It can beseen that the overlapping of the diamond pixels 40 generates diamondshaped interpolated pixels 44. The interpolated pixels have an areaequal to about 25% of the original pixels 40. The side dimension of thediamond shaped interpolated pixels 44 (which in a constructed exemplaryembodiment is equal to 0.0047 inches) is 0.707 (=1/square root of 2)times the side dimension of the traditional square pixel 42. For a givendata input, these smaller interpolated pixels 44 cause an increasedvisual resolution with a corresponding increase in apparent sharpness inthe final image.

[0034] The interpolated pixels 44 create an effective blending because,in part:

[0035] 1) The head is in motion as it exposes pixel images which causesthe amount of light delivered to the photosensitive material to begreatest where the pixel is widest in the scan direction, and

[0036] 2) The photosensitive material is generally not sufficientlyresolute to resolve a sharp image of one diamond pixel. Consequently thematerial diffuses adjacent pixels into each other to some degree.

[0037]FIG. 8 shows the spectral characteristics of typical red 64, green62 and blue 60 LED light sources. Due to manufacturing variations commonin LED technology, there can be considerable variation in the spectraloutput from similar LEDs. FIG. 9 shows a typical range of variation inspectral outputs in a small population of red, green and blue LED lightsources, respectively 64′, 62′ and 60′. FIG. 10 shows the narrow passspectral transmission characteristics 72, 74 of blue and green filtersdepicted in FIG. 7. By passing the energy of each of the LEDs in thepopulation of curves shown in FIG. 9, through a filter whose spectralresponse is shown in FIG. 10, the spectral output of each of the LEDs isconstrained to fall within the envelope of the filter. After filtering,all light sources will have substantially the same spectralcharacteristics and function substantially the same when exposing thedifferent colors of the photosensitive material.

[0038]FIG. 7 shows a preferred color matching filter 52 installed on theprint head 1. The color filter 52 is comprised of three sections: blue54, green 56 and red or clear 58. The blue section 54 of the colorfilter has the narrow pass spectral transmission characteristic 72 shownin FIG. 10. The green section 56 of the color filter has the narrow passspectral transmission characteristic 74 as shown in FIG. 10. The filter52 is positioned over the output ends of the fiber optic tubes so thatthe light supplied by blue light sources passes through the blue portion54 of the filter 52. Similarly, the light supplied by green lightsources passes through the green portion 56 of the filter 52. Lightsupplied by red light sources passes through the red (or clear) portion58 of the filter 52. In an embodiment of the invention constructed bythe inventors, the light supplied by the red light sources, can passthrough a substantially clear section of glass 58, i.e., the substratefor the blue 54 and green 56 filters because some photosensitivematerials have a very high tolerance to spectral variations in the redband.

[0039] The foregoing describes applicant's preferred method andapparatus for producing high quality continuous tone and/or color imageson photosensitive material. It is recognized that numerous modificationsand/or variations will occur to those skilled in the art withoutdeparting from the spirit or scope of the invention.

1. Apparatus for producing a color composite image on a target surfaceof photosensitive material extending in perpendicular lateral andlongitudinal directions, said apparatus comprising: a source of digitalsignals describing said composite image; an imaging head comprised of aplurality of light sources including a first group of light sources anda second group of light sources; said first group including a pluralityof light sources respectively exhibiting spectral outputs within a firstspectral band; said second group including a plurality of light sourcesrespectively exhibiting spectral outputs within a second spectral band;said imaging head being mounted proximate to said target surface; acontroller responsive to said digital signals for selectively energizingsaid light sources to image substantially identically shaped pixels ontosaid target surface to form said composite image, said composite imagebeing formed of multiple longitudinally displaced pixel rows whereineach row is comprised of laterally displaced pixel images; a firstfilter mounted between the light sources of said first group and saidtarget surface, said first filter exhibiting a narrow pass spectraltransmission characteristic within said first spectral band for causingsaid photosensitive material to respond substantially uniformly to saidlight sources of said first group; and; a second filter mounted betweenthe light sources of said second group and said target surface, saidsecond filter exhibiting a narrow pass spectral transmissioncharacteristic within said second spectral band for causingphotosensitive material to respond substantially uniformly to said lightsources of said second group;
 2. The apparatus of claim 1 wherein saidimaging head includes a third group of light sources; and wherein saidfirst, second, and third groups of light sources respectively producelight within the green, blue, and red spectral bands.
 3. A method forproducing a color composite image on a target surface of photosensitivematerial, said method including the steps of: providing a plurality offirst light sources each energizable to produce light having acharacteristic spectral output where said first light sourcescollectively define a first spectral range; providing a plurality ofsecond light sources each energizable to produce light having acharacteristic spectral output where said second light sourcescollectively define a second spectral range; passing the light producedby said first light sources through a first filter having a spectraltransmission characteristic narrower than said first spectral range toconstrain the outputs of said first light sources to the spectralcharacteristic of said first filter, passing the light produced by saidsecond light sources through a second filter having a spectraltransmission characteristic narrower than said second spectral range toconstrain the outputs of said second light sources to the spectralcharacteristic of said second filter; applying energizing signals tosaid first and second light sources to produce a composite image; andimaging said composite image onto a target surface of photosensitivematerial.
 4. The method of claim 3 wherein each of said plurality offirst light sources produces light within a green spectral range andeach of said plurality second light sources produces light within a bluespectral range.